There is a move to the use of charge coupled device arrays as the image receivers in spectrophotometers. Such charge coupled device arrays can replace single photodiodes, linear photodiode arrays and photomultiplier tubes. There is a potential advantage in using charge coupled device arrays because they can capture information on more than one spectral line simultaneously which is very useful when performing multi-element analysis using techniques such as inductively coupled plasma emission spectroscopy for the detection of metals in solution. Instruments using charge coupled device arrays are potentially very much faster for multi-element analysis since the information on all of the elements or species is captured simultaneously. As it is possible to capture information on more than one spectral line simultaneously more reliable comparative results can be obtained since all the elements being analysed and compared are analysed at the same time from exactly the same sample. When using a single detector as the image receiving element there is inevitably a time lag between the detection of the different species which involves a lack of certainty over whether the sample and the operating conditions are uniform over a period of time.
It is often desirable to monitor the intensity profile of a particular spectral line, especially when developing an analysis method, to examine the spectral line for line overlaps and hence interferences on the analytical measurements between a line distinctive of one element and interfering line from another element.
In the past when using a photomultiplier tube, for example, it is common to provide an exit slit which is matched to that of the entrance slit to isolate light from a single spectral line. To obtain information on the intensity profile of a spectral line, what has been done previously is to step the exit slit across the spectral line in, for example, steps of a tenth of the width of the exit slit. By subsequently analysing the results obtained at each step it is possible to produce a plot of intensity against wavelength and by examining the shape of this plot it is possible to determine whether the spectral line is "clean" or whether it includes shoulders or dips indicating the presence of light from an interfering species.
When using a charge coupled device array it would be desirable to have a charge coupled device array of sufficiently high resolution that each spectral line was spread over many pixels in the wavelength direction. In this way, the output of each pixel would correspond to each step of the conventional scan in the wavelength direction and would again enable an intensity profile of the spectral line to be determined. However, charge coupled device arrays of sufficient size and pixel density are not normally available and would, in any event, be prohibitively expensive. Charge coupled device arrays in which the pixel size corresponds to the full height half width of a spectral line and of a size so that the entire spectrum of a spectrometer can be imaged on it are much more reasonably priced. Accordingly, what has been proposed at present with such devices is to scan the charge coupled device array with respect to the spectral lines so that, for example, the spectral line is moved in the wavelength direction with respect to the charge coupled device array in steps of a tenth of the pixel width or spectral line full height half width. Thus, this is exactly analogous to the conventional relative movement of the exit slit and spectral line to be able to get an output giving information on the intensity of the spectral line with respect to wavelength.
This technique, however, has many of the disadvantages of a conventional instrument using only a single detector. Mechanical means to provide the relative movement between the spectral line and the charge coupled device array must be provided and their movement controlled and monitored accurately. It is not possible to carry out the monitoring of all spectral line profiles simultaneously in a single step because, inevitably, there is a time lag between each measurement position as the relative movement between the charge coupled device array and the spectral line occurs. Changes in sample concentrations or operating parameters during this period naturally affect the result and lead to the generation of false intensity distribution profiles. Whilst the use of a charge coupled device array is faster than the use of a single detector because it is possible to monitor several different lines simultaneously it is still not as fast as is potentially possible because it is still necessary to carry out a number of different reads of the charge coupled device array as it is scanned with respect to the spectral line for each sample to be analysed.